Internal combustion engines can have a plurality of cylinders, with the cylinders having one or more valves configured to facilitate the intake of an air-fuel mixture to the cylinder and/or the exhaust of the combusted air-fuel mixture from the cylinder.
In certain internal combustion engines, poppet-style valves are utilized to facilitate the intake and exhaust from the cylinder. Poppet-style valves typically have a head connected to a stem. Poppet-style valves are conventionally housed in a cylinder head positioned adjacent to the engine block. The cylinder head can includes any number of poppet-style valves associated with the cylinders of the engine block.
During operation of the engine, with the poppet-style valve in a closed position, the head of the valve seals against a valve seat positioned in the cylinder head. The stern extends from the head of the valve, through a guide in the cylinder head, and a distal end of the stem attaches to a spring device, typically a compression-type coil spring. The force exerted by the spring device on the stem maintains the valve in the closed position. As the engine runs, an opposing force is applied to the stem, thereby compressing the spring device and separating the valve head from the seat and opening the valve. Upon completion of the respective intake or exhaust step, the opposing force is removed from the stem and the spring device returns the valve head to its sealed position against the valve seat.
Due to repetitive compression and decompression cycles of a running engine, the spring device can be prone to fatigue related failure, especially when cycled at higher engine speeds. In addition to affecting performance of the engine, failure of a spring device may result in significant damage to the interior of the engine.
In response to failure issues of spring devices, pneumatic valve springs can be used. Unlike a mechanical valve spring device, which typically relies on the elasticity the spring material, a pneumatic valve spring relies on pressure of a compressed gas within a valve housing. Because the elastic element of the pneumatic valve spring is provided by compressed gas instead of the solid material, fewer fatigue related failures are realized. However, although pneumatic valve springs have reduced the possibility of suffering from fatigue related failure, they can be prone to other problems.
One such issue is that the pressure of the compressed gas within the pneumatic valve must be maintained in order to keep the pneumatic valve in a naturally closed position. Generally, the pressure of the compressed gas within the pneumatic valve is generated as the engine is running. However, if an engine is not operated for a period of time, the pressure of the compressed gas within the pneumatic valve can seep out of the pneumatic valve, and the pressure of the compressed gas can eventually decline to the extent that the pneumatic valve can fall from a sealed position against the valve seat to its lowest extent, typically in an open position. This could create engine damage upon startup.
To overcome the issue of declining pressure of the compressed gas during shutdown of the engine, some pneumatic valves have been modified to include a mechanical-type spring. In a first type of pneumatic-mechanical hybrid, a lightweight mechanical spring is disposed within the pneumatic spring. The mechanical spring is attached to both the top and bottom of the pneumatic spring, and maintains the pneumatic spring in an extended state when the pressure of the compressed gas within the pneumatic spring is lost. In this first type of pneumatic-mechanical hybrid spring, the mechanical spring cycles with the pneumatic spring during engine operation. In this arrangement, the possibility of a fatigue related valve spring failure is reintroduced.
In a second type of pneumatic-mechanical hybrid, the mechanical valve spring is attached to the top of the pneumatic spring. When the engine is not running, the mechanical spring extends to maintain the valve in a closed position, that is, the valve head is seated against the valve seat. This allows the valve train to operate normally during start-up of the engine. Once the engine is operating normally, the pressure of the compressed gas is supplied to the pneumatic valve and the mechanical spring becomes compressed during operation. Thus, the issue of fatigue related failure of the mechanical spring is eliminated.
Both the first and second types of pneumatic-mechanical hybrids are considered less than ideal, as the addition of the spring force, in the first type, or the addition of the spring mass, in the second type, increases the moving mass of the pneumatic valve, thereby reducing overall efficiency of the valve train and the engine.
Accordingly, there exists a need in the art for a valve assembly that eliminates fatigue related failure, maintains a spring force while the engine is inoperative, and does not increase the moving mass of the valve.